Cyclotrons are used for high-energy particle production. Cyclotron technology has been developed over many decades, and today it is considered a mature technology.
The present approach for making cyclotrons includes the use of magnetic iron poles and iron return yokes, to decrease the quantity of conductor needed to generate the magnetic field. In addition, magnetic iron poles are used for shaping the field. It is well known that the radial and azimuthal field profiles are essential for particle acceleration and for particle stability. For synchrocyclotrons, the axial field component needs to decrease with increasing radius to provide particle stability. For isochronous cyclotrons, the average magnetic field needs to increase to balance the increase in mass with energy of the particle due to relativistic effects, and the field must vary azimuthally to provide beam stability.
The use of superconductivity in cyclotrons opens the potential for compact, high-field devices, and external shielding may be needed to protect surrounding environments from high magnetic fields extending outside the cyclotron.
Yoke-free isochronous cyclotron concepts have been proposed in the past (see U.S. Pat. No. 4,943,781 (Martin N. Wilson, Martin F. Finian, “Cyclotron with Yokeless Superconducting Magnet”). The field shaping for the isochronous cyclotron was achieved using a combination of coils and iron pole tips in the bore of the coils, limiting the flexibility of field shaping by coils that are above/below the beam chamber. There is no mention of any means to minimize the stray magnetic field in this concept.